This invention relates to the reduction of color formation in diene polymers and hot melt adhesive formulations which contain such diene polymers, especially block copolymers of conjugated dienes and vinyl aromatic hydrocarbons. More particularly, it relates to the reduction of such color formation by the use of coupling agents whose by-products of reaction do not contribute to color formulation.
It is known that a block copolymer can be obtained by an anionic copolymerization of a conjugated diene compound and an alkenyl arene compound by using an organic alkali metal initiator. Block copolymers have been produced which comprise primarily those having a general structure EQU A--B and A--B--A
wherein the polymer blocks A comprise thermoplastic polymer blocks of alkenyl arenes such as polystyrene, while block B is a polymer block of a conjugated diene such as butadiene or isoprene. The proportion of the thermoplastic blocks to the elastomeric polymer block and the relative molecular weights of each of these blocks is balanced to obtain a rubber having unique performance characteristics. When the content of the alkenyl arene is small, the produced block copolymer is a so-called thermoplastic rubber. In such a rubber, the blocks A are thermodynamically incompatible with the blocks B resulting in a rubber consisting of two phases--a continuous elastomeric phase (blocks B) and a basically discontinuous hard, glass-like plastic phase (blocks A) called domains. Since the A--B--A block copolymers have two A blocks separated by a B block, domain formation results in effectively locking the B blocks and their inherent entanglements in place by the A blocks and forming a network structure.
These domains act as physical crosslinks anchoring the ends of many block copolymer chains. Such a phenomenon allows the A--B--A rubber to behave like a conventionally vulcanized rubber in the unvulcanized state and is applicable for various uses. For example, these network forming polymers are applicable for uses such as adhesive formulations; as moldings of shoe soles, etc.; impact modifier for polystyrene resins and engineering thermoplastics; modification of asphalt; etc.
Many such polymers are made by coupling with coupling agents as described in U.S. Pat. No. 4,096,203. Such coupling agents, including dibromoethane, may contain halogens. With halogen-containing coupling agents, the coupling reaction results in the formation of Lithium halide, LiX. This can cause the polymer to turn brown when the temperature is elevated, a disadvantage for applications such as films and coatings, especially when high clarity is required.
Adhesive formulations containing coupled polymers containing LiI, LiCl or LiBr (but not LiF) salts are known to develop a brown color when aged at high temperatures such as 177.degree. C. This problem does not occur in sequentially polymerized polymers. This is considered a problem for hot melt adhesive manufacturers who sometimes hold adhesives at high temperatures for extended times during hot melt application. The brown color is undesirable for many end uses for such adhesive products such as diaper assembly, clear labels, clear tapes, clear decals, etc.
We have found that color generation is not correlated with increased oxidative degradation. Thus, the commonly used methods for dealing with color formation in polymers are not effective in this situation. We have found that, the use of specific coupling agents and antioxidants will greatly reduce the color formation in these polymers and hot melt adhesive formulations containing them.